Dexterous manipulation with a highly redundant movement system is
one of the hallmarks of human motor skills. From numerous behavioral
studies, there is strong evidence that humans employ compliant
task space control, i.e. they focus control only on task variables while keeping redundant degrees-of-freedom as compliant as possible. This
strategy is robust towards unknown disturbances and simultaneously
safe for the operator and the environment. The theory of operational
space control in robotics aims to achieve similar performance properties.
However, despite various compelling theoretical lines of research,
advanced operational space control is hardly found in actual
robotics implementations, in particular new kinds of robots like humanoids
and service robots, which would strongly profit from compliant
dexterous manipulation. To analyze the pros and cons of different
approaches to operational space control, this paper focuses on
a theoretical and empirical evaluation of different methods that have
been suggested in the literature, but also some new variants of operational space controllers. We address formulations at the velocity,
acceleration, and force levels. First, we formulate all controllers in
a common notational framework, including quaternion-based orientation
control, and discuss some of their theoretical properties. Second,
we present experimental comparisons of these approaches on
a seven-degree-of-freedom anthropomorphic robot arm with several
benchmark tasks. As an aside, we also introduce a novel parameter
estimation algorithm for rigid body dynamics, which ensures physical
consistency, as this issue was crucial for our successful robot implementations.
Our extensive empirical results demonstrate that one of
the simplified acceleration-based approaches can be advantageous
in terms of task performance, ease of parameter tuning, and general
robustness and compliance in the face of inevitable modeling errors.

Example
4: Experiment (7) (“figure 8” movement
with orientation control at slow
speed) with the controller 5. A cup
containing water is placed on the
end-effector to demonstrate the effectiveness
of orientation control. (4.8 MB) mp4

5

Example
5: Manually applying perturbations to
the robot during the motion to show
the level of compliance and robustness
of the control using the controller
5). (3.9 MB) mp4